Twin-spindle inertia welding machine

ABSTRACT

An inertia welding machine includes a pair of axially movable spindle assemblies arranged in facing relation with a nonrotating fixture arranged between the two spindle assemblies. A single drive train including a flywheel is coupled to both of the spindle assemblies through a synchronizing shaft. Preferably, a clutch is arranged between the flywheel and the two spindles to allow very precise control over the final length of weld parts joined by the machine and one of the spindle assemblies is selected to have a slightly increased rotating mass to overcome a tendency for angular misalignment between the two spindle assemblies.

United States Patent [191 Franks et al.

Apr. 2, 1974 Inventors: Douglas G. K. Franks; Ira II. Sage,

both of Peoria, Ill.

10/1971 l-lallenberg 29/4703 X 76 Ill 56 49 3,727,298 4/1973 Gage et al228/2 X Primary ExaminerJ. Spencer Overholser Assistant Examiner-RobertJ. Craig Attorney, Agent, or FirmFryer, Tjensvold, Phillips & Lempio[57] ABSTRACT An inertia welding machine includes a pair of axiallymovable spindle assemblies arranged in facing relation with anon-rotating fixture arranged between the two spindle assemblies. Asingle drive train including a flywheel is coupled to both of thespindle assemblies through a synchronizing shaft. Preferably, a clutchis arranged between the flywheel and the two spindles to allow veryprecise control over the final length of weld parts joined by themachine and one of the spindle assemblies is selected to have a slightlyincreased rotating mass to overcome a tendency for angular misalignmentbetween the two spindle assemblies.

7 Claims, 7 Drawing Figures PATENTED APR 2 I974 SHEET 5 {IF 6 Pmmemrk2:914"

saw an? e BACKGROUND OF THE INVENTION The present invention relates toan inertia welding machine and more particularly to such a machinehaving opposed rotatable spindle assemblies with a nonrotating fixturemounted between the spindles to support either a single weld part or twoseparate weld parts in axial alignment with weld parts mounted in thespindle assemblies.

Prior art inertia welding machines have included axially aligned holdingfixtures for simultaneously accomplishing multiple bonds. However, thesemachines are relatively limited in terms of welding applications forwhich they are suited.

The present invention is intended to provide a versatile inertia weldingmachine capable of performing inertia welds in a number of differentapplications.

SUMMARY OF THE INVENTION The twin-spindle inertia welding machine of thepresent invention includes a pair of rotatable spindle assembliesmounted in facing relation with a non-rotating fixture disposedtherebetween. With such an arrangement, the non-rotating fixture may beadapted to secure a single weld piece or a pair of separate weld piecesin axial alignment with the two spindle assemblies.

To simplify operation of the machine in accomplishing a bond at twointerfaces, a single drive train including a flywheel capable ofdelivering a substantial amount of the rotational energy required forboth spin dles is coupled with the two spindles through a synchronizingshaft. Preferably, the synchronizing shaft is constructed in twoportions which are secured together by a coupling capable of allowingangular adjustment of the two shaft portions.

A clutch is preferably arranged between the single drive train and thetwo spindles to allow for very precise control over the finished lengthof a bonded weld piece in a manner described in greater detail below.

Other objects and, advantages of the present invention are madeiapparentin the following description having reference to the accompanyingdrawings.

BRIEF DESCRIPTION OF THE DRAWINGS FIG. 1 is a side view in elevation ofatwin-spindle inertia welding machineconstructed according to the presentinvention.

FIG. 2 is a plan view of the machine illustrated in FIG. 1.

FIG. 3 is a fragmentary side view, with parts in section, of a drivetrain which also is shown at the left end of FIGS. 1 and 2.

FIG. 4 is a view taken along section line IV- IV of FIG. 1 to moreclearly illustrate the construction of one of the spindle assemblies inthe machine.

FIG. 5 is a view taken along section line VV of FIG. 4 to illustrateinternal components of the spindle assembly.

FIG. 6 is an enlarged fragmentary view, with parts in section, of afixed tailstock assembly associated with one of the spindle assemblies.

FIG. 7 is a generally schematic representation, with parts in section,of the single drive train, synchronizing shaft and spindles to moreclearly illustrate the manner 2 I in which the single drive train iscoupled with the two rotatable spindle assemblies of the machine ofFIGS. 1 and 2.

DETAILED DESCRIPTION OF A PREFERRED EMBODIMENT A twin-spindle inertiawelding machine constructed according to the present invention isindicated at 10 in FIGS. 1 and 2. The twin-spindle machine has a base orframe structure 11 for supporting a single drive train 12 and twospindle assemblies 13 and 14 which are arranged in facing relation witheach other. A central holding fixture 15 having dual non-rotatable chuckassemblies 15a and 15b is mounted upon the train structure 11 betweenthe two spindle assemblies. The two spindle assemblies 13 and 14 areaxially movable in a manner described below while the fixture 15 ispreferably secured in place upon the frame structure 11.

Two upright tailstock supports 18 and 19 are secured to opposite ends ofthe frame 11. Three horizontal tie bars 20, 21 and 22 extendbetween andprovide triangular supports for the upright supports 18 and 19. Aplurality of tie bar supports 26 are also arranged in spaced apartrelation on the frame 11 to support the tie bars 21 and 22. Both of thespindle assemblies 13 and 14 are slidably supported upon the tie bars 21and 22 for axial movement relative to the central assembly 15.

Axial movement of the two spindle assemblies is controlled bydouble-acting hydraulic rams 27 and 28 associated respectively with theupright supports 18 and 19 and operatively coupled with the spindleassemblies 13 and 14 respectively. The double-acting ram 27 is securedto the support 18 and has a piston rod 29 coupled to the spindleassembly 13. Similarly, the hydraulic ram 28 has a piston rod 30 whichis secured to the spindle assembly 14.

Weld pieces 32 and 33 are secured for rotation by chuck assemblies 16and 17 respectively mounted upon the spindle assemblies 13 and 14. Anon-rotatable weld piece 34 is shown secured by the fixture 15. Althoughthe non-rotating weld piece 34 is shown as a single part, it may also bereplaced by two separate non-rotatable weld pieces (not shown) whichwould be secured in the dual holding fixtures 15a and 15b of theassembly 15. During a welding operation, the rotatable weld pieces 32and 33 are moved into abutting engagement with opposite ends of thenon-rotatable weld piece 34 by movement of the spindle assemblies 13 and14 along the tie bars. The machine might be made somewhat more versatileby mounting the non-rotatable fixture 15 for axial movement upon thesupporting frame 11. However, for purposes of the present embodiment,the assembly 15 is contemplated as being fixed in place upon the frame11.

The single drive train 12 comprises a hydrostatic transmission includinga hydraulic motor as generally indicated at 35 and a gear train arrangedwithin a housing 39 for coupling the motor 35 with a flywheel shaft 36(See FIG. 1). The rotatable spindle assemblies are coupled together by asynchronizing shaft 37 which is constructed in two parts, 37a and 37b,which are secured together near the tailstock fixture 15 by anadjustable coupling 38. The flywheel shaft 36 is coupled to thesynchronizing shaft 37a by a hydraulically actuated clutch 41. Theclutch 41 is adapted to selectively uncouple the single drive train 12from both spindle assemblies 13 and 14 for a purpose discussed ingreater detail below.

' The drive train 12 is shown in greater detail in FIG. 3 with the geartrain being generally indicated at 40 for coupling the motor with theflywheel shaft'36. The gear train 40 consists of meshing drive gears42-49. The first drive gear 42 is coupled to an output shaft 51 for themotor 35. The gear 43 is arranged in meshing relation between the gear42 and another gear 44 which is mounted upon a gear shaft 52 along withanother gear 45. The gear 45 in turn meshes with a gear 46 which ismounted upon another gear shaft 53 along with the drive gear 47 whichmeshes with the final drive 'gear 49 through the drive gear 48. The lastdrive gear 49 is secured upon the flywheel shaft 36.

Versatility within the drive train is provided by replaceable mountingofthe flywheel 50 upon a flanged portion 56 of the flywheel shaft 36.Additionally, drive gears 45 and 46 are change gears which can bereadily replaced by different gears to provide a different drive ratiowithin the gear train 40. Access to the gears 45 and 46 is facilitatedby a removable cover plate 57 v mounted uponthe housing 39.

The clutch assembly 41, as best seen in FIG. 3, provides a selectivecoupling between the flywheel shaft 36 and an external clutch housing 61which has an axially extending shaft portion 61a rotatably mounted in asupport block 63 by bearing means 62. The shaft 61a is secured to aspacer shaft 65 by means of a coupling 64. A key 66 penetrates thecoupling 64 and shaft 65 to provide a positive drive connectiontherebetween. The other end of the spacer shaft 65 is coupled to thesynchronizing shaft 37a by a second coupling 71 and key 72 asillustrated in FIG. 5 and described in greater detail below.

- Referring particularly to FIGS. 4 and 5, each of the spindleassemblies 13 and 14 has a housing 68 rotatably mounting a spindle 69.Each spindle 69 is coupled to the synchronizing shafts 37a or 37b bymeans ofa spindle gear 70, idler gears 74 and 75 and a drive gear 76. Acoupling is provided between the spindle chuck 16 and the shaft 370, forexample, by means of intermediate gears 74 and 75 which mesh with thespindle gear 70 and the drive gear 76 which is splined upon the shaft37a as indicated generally at 82.

The spindle 69 contains a clamping cylinder 77 and piston 78reciprocably arranged within the cylinder forcontrolling engagement ofthe chucks l6 and 17 with the respective rotatable weld pieces. Clampingengagement of the chuck is provided by actuating fluid introduced intothe cylinder 77 by way of an external fitting 80 while the chuck isdisengaged by means of fluid introduced into the cylinder throughanother external fitting 81.

A hydrostatic bearing 84 is formed between opposed faces of a thrustbushing 85 and a shaft member 86 for absorbing and transferring weldingthrust forces. A controlled forward leakage path 88 and a controlledrearward'leakage path 89 are provided for the bearing chamber 84 withhydraulic fluid being introduced into the bearing chamber 84 through aninlet port 90 formed in the spindle housing 68.

A rear housing 91 provides a closure for the spindle interior whilecoupling the piston rod 29 with the spindle assembly 13. Accordingly,the entire spindle assembly 13 is axially shifted along the supportingtie bar 20-22 by extension or retraction of the piston rod 29.

v The spindle 13 remains in rotating engagement with the synchronizingshaft 37a during its axial movement along the tie bars since thesynchronizing shafts are splined along the entire length of travel forthe respecthe spindle assembly 13. The hydraulic ram 27 comprises arapid advance cylinder 95 containing a'piston .96 which is secured tothe piston rod 29. A thrust cylinder 97 is formed in the upright support18 with a piston 98 being reciprocably mounted therein. Referringmomentarily to FIG. 5, the piston rod 29 is secured to the rear housing91 of the spindle assembly 13 by means of a threaded stud 99.

Versatility of the welding machines is increased by the provision ofspacers 100 and 101 attached respectively to the back of the rapidadvance piston 96 (opposite piston rod 29) and to the front of the rapidadvance piston 96 to encircle the piston rod 29. The spacer 100 limitsreverse travel of the piston 96 and accordingly reduces cycle time foroperating the machine. The axial dimension of the spacer is of courseselected according to the length of the weld parts secured with themachine. The other spacer 101 limits forward travel of the piston 96,thus increasing the distance between the spindle position illustrated inFIG. 1 and the weld position with the weld pieces brought into axialengagement thus permitting weld parts of different lengths to be weldedupon the machine.

It is noted that the position of the upright supports 18 and 19 arefixed relative to the tie bars 2022 by plate members 102 which aresecured to the respective upright supports for example by bolts 103 andto the respective tie bars for example by bolts 104. Actuating fluid isintroduced into the cylinder 95 by means of inlet conduits 105 and 106.Fluid introduced through the conduit 105 tends to extend the rod 29 thusshifting the spindle 13 toward the fixed mounting assembly 15 (SeeFIG. 1) while fluid is introduced through the conduit 106 to retract therod 29.

The construction and interconnection of the spindle assembly 14 with thesynchronizing shaft 37b and tailstock 19 is similar to that describedabove for the spindle assembly 13, the synchronizing shaft 37a and thetailstock 18.

According to the previous description, it is noted that the spindleassemblies 13 and 14 are respectively driven by the splinedsynchronizing shaft portion 37a and 37b respectively through drive gears70, 74, 75 and 76. It is, of course, necessary to provide for a limitedamount of back lash or clearance between the gears and also in thesplined couplings. It is further noted that angular correlation betweenthe two spindle assemblies 13 and 14 may be more important in someapplications than in others. Since the spindle assemblies 13 and 14 willdecelerate according to prevailing frictional torque developed at therespective weld interfaces, that is, between weld pieces 32, 34 and 33,34, angular misalignment of the spindle assemblies 13 and 14 may result.As one solution to overcoming this problem, it is contemplated toprovide one or the other of the spindle assemblies with an extra orvariable motational moment of inertia. Although the moment of inertiafor one of the spindles, for example, that indicated at 14, ispreferably contemplated as merely having a slightly increased mass withrespect to the other spindle 13, it is also noted thata variable momentof inertia could be provided for one of the spindles l3 and 14. Thespindle having a larger moment of inertia will tend to deceleratesomewhat slower than the other spindle, thereby tending to preventbacklash in the drive train. The coupling 38 which may be of anyconventional design to allow for angular adjustment of the shaftportions 37a and 37b can then be employed to angularly adjust thesynchronizing shaft portions 37a and 37b in order to compensate for thebacklash. To further increase versatility of the machine, it iscontemplated that a light auxiliary flywheel, 113 or 114, could beemployable respectively with the spindles 13 and 14.

The preceding description of the single drive train for driving therespective spindles 13 and 14 through the synchronizing shafts 37a and37b may be more readily seen by reference to FIG. 7. Mounting blocks 111for supporting the shaft portions 37a and 37b are also best seen in FIG.7. i

In a preferred manner of operating the twin-spindle inertia weldingmachine as described above, anonrotatable weld piece 34 is first securedwithin the holding fixture 15. Rotatable weld pieces 32 and 33 are alsosecured within the rotatable chucks 16 and 17. Actuat ing fluid is thenintroduced into the rapid advance cylinders 95 (through inlet conduit105) to move the two spindle assemblies 13 and 14 toward thenon-rotatable fixture corresponding to a prebond position for themachine. Once the spindles are in this position with the weld piecesaxially adjacenteach other, the drive train 12 for the machine may beactuated by operating the motor 35.

With the clutch 41 engaged, the flywheel 50 is brought up toa selectedspeed of rotation by the motor 35 acting; through the drive train 40.The selected speed of rotation for the flywheel is preferably selectedto provide adequate energy for subsequently driving the spindleassemblies 13 and 14 to affect a completed bond at weld interfacesbetween the weld pieces 32, 34 and 33, 34. V

When'the flywheel is rotating at the selected speed, operation of themotor 35 is discontinuedand, at approximately the same time, actuatingfluid is introduced into the thrust cylinders 97 through inlet ports 109in order to shift the spindle assemblies 13 and 14 toward thenon-rotatable fixture 15 and axially engage the weld piecesundersufficient pressure to accomplish a bond therebetween. Welding energy issupplied from the flywheel 50 and as stored energy from the flywheel isconverted into heat at interfaces between the weld pieces, rotatingspeed of the spindles 13 and 14 and the drive train 12'is rapidlyreduced. Prior to stopping of the spindles, however, the clutch 41 isdisengaged to interrupt the driving connection between the flywheelshaft 36 and the spindle assemblies 13 and 14. At this point in time,there is little inertia remaining in the rotating spindle assemblies andassociated component. Accordingly. rotation of the spindles 13, 14 andtheir associated weld pieces is rather abruptly stopped once the clutch41 is disengaged to allow for very precise control over the length ofthe bonded Weld piece. In the operation described above, the finishedweld piece would include the non-rotating weld piece 34 and bothrotatable weld pieces 32 and 33.

Once the weld is completed, fluid may be introduced into the cylinders97 through inlet ports 110 to retract the thrust piston 98. Similarly,fluid is introduced into the rapid advance cylinders through inlet ports106 to retract the spindles l3 and 14 to their positions shown inFIG. 1. In this position, after new weld pieces have been placed in thechucks 16, 17 and the holding fixture 15, a new holding operation may becommenced.

What we claim is:

1. A twin-spindle inertia welding machine comprising a frame structure,each of the spindle assemblies having a rotatable chuck for securing arespective weld piece, the rotatable chucks being arranged upon therespective spindle assemblies in facing relation with each other.

a non-rotating holding fixture mounted upon the frame structure betweenthe spindle assemblies with means effectivelyinterconnecting thespindles and holding fixture for positioning the two spindles relativeto the holding fixture,'the spindle chucks and holding fixture beingadapted to respectively receive rotatable and non-rotatable weld piecesin axial alignment,

drive means arranged adjacent one of the spindle assemblies having arotatable flywheel, motor means for setting the flywheel in rotation anda single output shaft, the motor means being arranged closer to one ofthe spindle assemblies than the other spindle assembly, and

synchronizing means including an elongated shaft assembly for couplingthe output shaft with the two spindles so that energy for driving thespindles in rotation is substantially provided by the flywheel,

one of the chucks having a relatively larger moment of inertia than theother chuck for overcoming angular misalignment, between the tworotatable weld pieces as caused by the relative position of the motormeans and its interconnection with the two spindles by the single outputshaft.

2. The welding machine of claim 1 wherein the elongated shaft assemblyof the synchronizing means comprises twoshaft portions coupledrespectively to the spindle assemblies and to each other by means of acoupling capable of allowing angular adjustment between i the two shaftportions.

3. A twin-spindle inertia welding machine comprising a frame structure,a pair of rotatable spindle assemblies mounted upon the frame structure,each of the spindle assemblies having a rotatable chuck for securing arespective weld piece, the rotatable chucks arranged upon the respectivespindle assemblies in facing relation with each other,

non-rotating holding fixture mounted upon the frame structure betweenthe spindle assemblies with means effectively interconnecting thespindles and holding fixture for positioning the two spindles relativeto the holding fixture, the spindle chucks and holding fixture beingadapted to respectively receive rotatable and non-rotatable weld piecesin axial alignment, drive means having a rotatable flywheel, motor meansfor setting the flywheel in rotation and a single output shaft, and

synchronizing means including an elongated shaft assembly for couplingthe single output shaft with the two spindles so that energy for drivingthe spindles in rotation is substantially provided by the flywheel, theelongated shaft assembly of the synchronizing means comprising two shaftportions coupled respectively to the spindle assemblies and to eachother by means of a coupling capable ofallowing for angular adjustmentbetween the two shaft portions.

4. A twin-spindle inertiawelding machine comprising a frame structure,

a pair of rotatable spindle assemblies both mounted for axial movementupon the frame structure, each of the spindle assemblies having arotatable chuck for securing a respective weld piece, the rotatablechucks being arranged upon the respective spindle I assemblies in facingrelation with each other,

a non-rotating holding fixture mounted in fixed relation upon the framestructure between the spindle assemblies with means effectivelyinterconnecting the spindles and holding fixture for positioning the twospindles relative to the holding fixture, the spindle chucks and'holdingfixture being adapted I to respectively receive rotatable andnon-rotatable weld pieces in axial alignment,

single drive means having a rotatable flywheel, motor means for settingthe flywheel in rotation and a single output shaft,

synchronizing means including an elongated shaft assembly forcouplingthe single output shaft with the two spindles so-that energy for drivingthe spindles in rotation is substantially provided by the flywheel, anda tailstock fixture associated with each of the movable spindleassemblies and secured in place upon the frame structure, a doubleactinghydraulic ram being interconnected between each'tailstock assembly andthe associated spindle assembly,

the drive means being arranged at one end of the frame structureadjacent one of the tailstock assemblies, the output shaft of the drivemeans being coupled to the synchronizing means by a clutch which isoperable to selectively couple and uncouple the drive means from the twospindles.

5. A twin-spindleinertia welding machine of the type wherein energy fordriving relatively rotatable weld pieces is substantially supplied by aflywheel means which is'set in rotation at a preselected speed by motormeans, comprising a frame structure,

a pair of spindle assemblies mounted upon the frame structure, each ofthe spindle assemblies having a rotatable chuck for securing arespective weld piece, the rotatable chucks arranged upon the respectivespindle'assemblies in facing relation with each other,

a non-rotatable holding fixture mounted upon the frame structure betweenthe spindle assemblies, means providing for axial movement between thespindles and the holding fixture,

an output shaft coupled to the flywheel means,

a synchronizing shaft connected to the output shaft and at itsopposite-ends to the respective spindle assemblies, the synchronizingshaft comprising two separate portions respectively coupled with the twospindle assemblies, the shaft portions being interconnected by means ofa coupling allowing for angular adjustment between the two shaftportions, and

a clutch arranged between the output shaft and the synchronizing shaftforselectively coupling and uncoupling the two spindles from theflywheel means.

6. The welding machine of claim 5 wherein the spindle assemblies areprovided with different moments of inertia.

7. A twin-spindle inertia welding machine of the type wherein energy fordriving relatively rotatable weld pieces is substantially supplied byflywheel means which are set in rotation at a preselected speed by motormeans, comprising a frame structure,

a pair of spindle assemblies mounted upon the frame structure, each ofthe spindle assemblies having a rotatable chuck for securing arespective weld piece, the rotatable chucks being arranged upon therespective spindle assemblies in facing relation with each other, thespindle assemblies being provided with different moments of inertia, forover coming angular misalignment between the two rotatable weld pieces,

a non-rotatable holding fixture mounted upon the frame structure betweenthe spindle assemblies,

means providing for axial movment between the spindles and the holdingfixture,

an output shaft coupled to the flywheel means.

a synchronizing shaft connected to the output shaft and at its oppositeends to the respective spindle assemblies, and

a clutch arranged between the output shaft and the synchronizing shaftfor selectively coupling and uncoupling the two spindles from theflywheel means.

1. A twin-spindle inertia welding machine comprising a frame structure,each of the spindle assemblies having a rotatable chuck for securing arespective weld piece, the rotatable chucks being arranged upon therespective spindle assemblies in facing relation with each other. anon-rotating holding fixture mounted upon the frame structure betweenthe spindle assemblies with means effectively interconnecting thespindles and holding fixture for positioning the two spindles relativeto the holding fixture, the spindle chucks and holding fixture beingadapted to respectively receive rotatable and non-rotatable weld piecesin axial alignment, drive means arranged adjacent one of the spindleassemblies having a rotatable flywheel, motor means for setting theflywheel in rotation and a single output shaft, the motor means beingarranged closer to one of the spindle assemblies than the other spindleassembly, and synchronizing means including an elongated shaft assemblyfor coupling the output shaft with the two spindles so that energy fordriving the spindles in rotation is substantially provided by theflywheel, one of the chucks having a relatively larger moment of inertiathan the other chuck for overcoming angular misalignment between the tworotatable weld pieces as caused by the relative position of the motormeans and its interconnection with the two spindLes by the single outputshaft.
 2. The welding machine of claim 1 wherein the elongated shaftassembly of the synchronizing means comprises two shaft portions coupledrespectively to the spindle assemblies and to each other by means of acoupling capable of allowing angular adjustment between the two shaftportions.
 3. A twin-spindle inertia welding machine comprising a framestructure, a pair of rotatable spindle assemblies mounted upon the framestructure, each of the spindle assemblies having a rotatable chuck forsecuring a respective weld piece, the rotatable chucks arranged upon therespective spindle assemblies in facing relation with each other, anon-rotating holding fixture mounted upon the frame structure betweenthe spindle assemblies with means effectively interconnecting thespindles and holding fixture for positioning the two spindles relativeto the holding fixture, the spindle chucks and holding fixture beingadapted to respectively receive rotatable and non-rotatable weld piecesin axial alignment, drive means having a rotatable flywheel, motor meansfor setting the flywheel in rotation and a single output shaft, andsynchronizing means including an elongated shaft assembly for couplingthe single output shaft with the two spindles so that energy for drivingthe spindles in rotation is substantially provided by the flywheel, theelongated shaft assembly of the synchronizing means comprising two shaftportions coupled respectively to the spindle assemblies and to eachother by means of a coupling capable of allowing for angular adjustmentbetween the two shaft portions.
 4. A twin-spindle inertia weldingmachine comprising a frame structure, a pair of rotatable spindleassemblies both mounted for axial movement upon the frame structure,each of the spindle assemblies having a rotatable chuck for securing arespective weld piece, the rotatable chucks being arranged upon therespective spindle assemblies in facing relation with each other, anon-rotating holding fixture mounted in fixed relation upon the framestructure between the spindle assemblies with means effectivelyinterconnecting the spindles and holding fixture for positioning the twospindles relative to the holding fixture, the spindle chucks and holdingfixture being adapted to respectively receive rotatable andnon-rotatable weld pieces in axial alignment, single drive means havinga rotatable flywheel, motor means for setting the flywheel in rotationand a single output shaft, synchronizing means including an elongatedshaft assembly for coupling the single output shaft with the twospindles so that energy for driving the spindles in rotation issubstantially provided by the flywheel, and a tailstock fixtureassociated with each of the movable spindle assemblies and secured inplace upon the frame structure, a double-acting hydraulic ram beinginterconnected between each tailstock assembly and the associatedspindle assembly, the drive means being arranged at one end of the framestructure adjacent one of the tailstock assemblies, the output shaft ofthe drive means being coupled to the synchronizing means by a clutchwhich is operable to selectively couple and uncouple the drive meansfrom the two spindles.
 5. A twin-spindle inertia welding machine of thetype wherein energy for driving relatively rotatable weld pieces issubstantially supplied by a flywheel means which is set in rotation at apreselected speed by motor means, comprising a frame structure, a pairof spindle assemblies mounted upon the frame structure, each of thespindle assemblies having a rotatable chuck for securing a respectiveweld piece, the rotatable chucks arranged upon the respective spindleassemblies in facing relation with each other, a non-rotatable holdingfixture mounted upon the frame structure between the spindle assemblies,means providing for axial movement between the spindles and the holdingfixture, an output shaft coupled to the flywheel means, a synchronizingshaft connected to the output shaft and at its opposite ends to therespective spindle assemblies, the synchronizing shaft comprising twoseparate portions respectively coupled with the two spindle assemblies,the shaft portions being interconnected by means of a coupling allowingfor angular adjustment between the two shaft portions, and a clutcharranged between the output shaft and the synchronizing shaft forselectively coupling and uncoupling the two spindles from the flywheelmeans.
 6. The welding machine of claim 5 wherein the spindle assembliesare provided with different moments of inertia.
 7. A twin-spindleinertia welding machine of the type wherein energy for drivingrelatively rotatable weld pieces is substantially supplied by flywheelmeans which are set in rotation at a preselected speed by motor means,comprising a frame structure, a pair of spindle assemblies mounted uponthe frame structure, each of the spindle assemblies having a rotatablechuck for securing a respective weld piece, the rotatable chucks beingarranged upon the respective spindle assemblies in facing relation witheach other, the spindle assemblies being provided with different momentsof inertia, for overcoming angular misalignment between the tworotatable weld pieces, a non-rotatable holding fixture mounted upon theframe structure between the spindle assemblies, means providing foraxial movment between the spindles and the holding fixture, an outputshaft coupled to the flywheel means, a synchronizing shaft connected tothe output shaft and at its opposite ends to the respective spindleassemblies, and a clutch arranged between the output shaft and thesynchronizing shaft for selectively coupling and uncoupling the twospindles from the flywheel means.